Forgivable Sins

Amen, I say to you, all sins and all blasphemies
that people utter will be forgiven them.
But whoever blasphemes against the Holy Spirit
will never have forgiveness,
but is guilty of an everlasting sin.”

Homilists, students of Sacred Scripture, and the casual observer all want to know: what is this unforgivable sin? What is this sin against the Holy Spirit?

I have my theories, but I think there is something more important going on here: everything else is forgivable!